Generally employed electrophotographic developers for visualizing an electrostatic latent image formed on an electrophotographic photoreceptor include one-component developers comprising a toner prepared by melt-kneading a binder resin, such as polystyrene, a styrene-butadiene copolymer or polyester, and a colorant or a dye, e.g., carbon black or Phthalocyanine Blue, followed by grinding and two-component developers composed of the toner and a carrier having approximately the same particle size as the toner or a particle size of up to 500 .mu.m, such as glass beads, iron particles, nickel particles, ferrite particles, etc. which may be coated with a resin.
When used as such, these developers are insufficient in performance properties, such as preservability (antiblocking properties), fluidity, developing properties, transfer properties, and charging properties, Therefore, external additives for improving these characteristics are often added to the toner. External additives proposed to date include hydrophobic silica fine powder (see JP-A-52-30437, the term "JP-A" as used herein means an "unexamined published Japanese patent application"), silica fine powder combined with aluminum oxide or titanium dioxide fine powder (see JP-A-60-238847), gas-phase-processed titanium dioxide having been rendered hydrophobic (see JP-A-59-52255), anatase titanium dioxide (see JP-A-60-112052), and aluminum oxide-coated titanium dioxide (see JP-A-57-79961). It has also been suggested to subject an inorganic compound proposed as an external additive to a surface treatment as exemplified by titanium dioxide fine particles having been surface-treated with a coupling agent as disclosed in JP-A-4-40467 and JP-A-4-348354.
Electrophotographic image formation basically consists of charging of a photoreceptor made of, for example, selenium, cadmium sulfide, zinc oxide, amorphous silicon, an organic photoconductor, etc., exposure to light, development, transfer, fixing, and cleaning. Of these processes, the conventional charging process is mostly carried out by applying a high voltage (DC 5 to 8 kV) to a metallic wire(s) to induce a corona discharge. The corona discharge involves the following disadvantages. Corona discharge products, such as ozone and NO.sub.x, denature the surface of a photoreceptor to cause image blurs or deterioration of the photoreceptor, and contamination of the wire gives adverse influences on image quality, causing image defects, such as white spots or black streaks. Further, a corona discharger is of low efficiency because most of the corona current passes through a grounding shielding plate with only 5 to 30% of the corona current being supplied to a photoreceptor.
In order to overcome these problems associated with corona discharge, the latest studies have been directed to contact charge using no corona discharger as described in JP-A-63-208881, JP-A-1-179578 and JP-A-64-73365. The charging process by contact charge is carried out by applying a direct voltage (V.sub.DC) and an alternating voltage (V.sub.AC) to a charging member, e.g., a conductive elastic roller, and bringing the charging member with an applied pulsating current (V.sub.DC +V.sub.AC) into contact with the surface of a photoreceptor to uniformly charge the photoreceptor to a prescribed potential.
Addition of a hydrophobic fine powder, such as silica fine powder, which is often used as an external additive to a toner, achieves considerable improvements in preservability, fluidity, developing properties, transfer properties, and the like. However, the hydrophobic fine powder, when added in an amount sufficient to exhibit desired improvements, adversely affects the charging properties of the toner. That is, of various charging properties required including quantity of charge, charging speed, distribution of charge, toner admixing properties, and environmental stability of charging, the charging speed, charge distribution, toner admixing properties, and environmental charging stability are adversely influenced by addition of silica, etc.
On the other hand, the combined use of alumina or titanium dioxide fine particles with silica fine powder in an attempt to improve the charging speed, charge distribution, toner admixing properties, and environmental charging stability gives rise to a problem that the quantity of charge attained is appreciably reduced. Moreover, the tolerance of a toner for these additives for satisfying the above-mentioned various charging properties is very narrow, and the achieved improving effects are not sufficient. In particular, environmental stability of charge quantity is still inferior.
Since the conventional gas-phase-processed rutile titanium dioxide or anatase titanium dioxide as non-treated are hardly charged, they must be coated with, e.g., alumina, or made hydrophobic by treating with a coupling agent for use as an additive for a toner. However, alumina-treated titanium dioxide particles are not always prevented effectively from agglomeration, and also their dispersibility in toner particles is unsatisfactory. Further, coupling agent-treated hydrophobic titanium dioxide particles are effective to some extent to improve charging properties but still insufficient.
While hydrophobic additives are generally prepared by a surface treatment with a silane coupling agent, those in which a hydroxyl group on the surface of silica particles is reacted with a silanol group derived from a silane coupling agent do not have sufficient hydrophobic nature and still need improvements in rise, uniformity and stability of charge and toner fluidity.
It is also known to effect charging by making use of the polarity of the treating agents. For example, a toner containing an aminosilane-treated metal oxide powder exhibits strong positive chargeability as disclosed in JP-A-52-135739 and JP-A-56-123550. However, aminosilane, being hydrophilic, aminosilane causes changes of toner fluidity or chargeability with time under a high temperature and high humidity condition.
Toners containing both aminosilane and a metal oxide powder having been rendered hydrophobic are proposed in JP-A-58-216252, JP-A-63-73271, and JP-A-63-73272, but they need improvements in rise and stability of charge and toner fluidity.
JP-A-60-93455 teaches a method for obtaining a negatively chargeable toner, in which an inorganic fine powder is surface-treated with a fluorine-substituted silane coupling agent having a negative polar group so that the chargeability of the polar group present on the surface of the inorganic fine powder is utilized for increasing the charge level of the toner. According to this technique, the resulting toner surely secures an increased charge level but is still unsatisfactory in terms of rise and stability of charge and toner fluidity. The publication refers to the particular case of obtaining negative chargeability by using no charge control agent or by using a reduced amount of a polyester resin as a binder resin of toner particles because the polyester resin per se possesses negative chargeability. Nevertheless, such a toner has a disadvantage of large dependence of chargeability on the environment, i.e., a great difference in quantity of charge between a high temperature and high humidity condition and a low temperature and low humidity condition. The disadvantage is especially conspicuous when a pigment other than carbon black is used as a colorant.
While not clear, the negative chargeability of a polyester resin seems attributed to the polar carbonyl ester group thereof. The chargeability of this polar group is susceptible to influences of changes in temperature and humidity, and the same applies where a polyester resin is used in a toner. Further, even if a charge control agent be added to a polyester resin, such cannot bring about any appreciable effect of reducing the variation of chargeability with changes in temperature and humidity. Thus, conventional toners containing a polyester resin as a binder resin fail to satisfy the requirements, such as environmental stability of charging properties.
On the other hand, where the above-mentioned contact charge is applied to electrophotography using a conventional toner composition, it turned out that residual trace amounts of toner particles and a released external additive, etc. on a photoreceptor after cleaning are deposited on the contact charging member, ultimately resulting in occurrence of image defects.